In recent years, in work machines such as hydraulic excavators and wheel loaders, energy saving has become a major development item in terms of so-called energy problems. Work machines of this kind use a hydraulic drive system for hydraulically driving a working section such as a front working device, and thus, the energy saving of the hydraulic drive system itself is important. As the hydraulic drive system, there has been known a hydraulic circuit referred to as a so-called closed circuit, wherein connection is made annularly (in a closed-circuit fashion) so that hydraulic fluid from a hydraulic pump being a pressure generation source is fed directly to a single-rod cylinder being a hydraulic actuator and that hydraulic fluid after used in performing a desired work by driving the single-rod cylinder is returned directly to the single-rod cylinder.
On the other hand, in contrast with the closed circuit, there has also been known a hydraulic circuit referred to as a so-called open circuit, in which hydraulic fluid is fed from a hydraulic pump to a single-rod cylinder through a restrictor configured by a control valve and in which the hydraulic fluid (return hydraulic fluid) that flows out from the single-rod cylinder is drained into a reservoir. Compared with the hydraulic circuit in the open-circuit fashion, the hydraulic circuit in the closed-circuit fashion is advantageous in fuel consumption performance because a pressure loss caused by a restrictor is little and because regeneration by the hydraulic pump is possible with the energy that the return hydraulic fluid from the single-rod cylinder possesses.
Further, Patent Literature 1 discloses prior art in which closed circuits of this kind are combined. In Patent Literature 1, there is installed a first closed circuit in which a hydraulic pump is connected to a boom cylinder being a single-rod cylinder in a closed circuit fashion, and there is also installed a second closed circuit in which a hydraulic pump is connected to an arm cylinder in a closed circuit fashion. Furthermore, an open circuit is installed in which a hydraulic pump is connected to a bucket cylinder through a control valve, and a distribution circuit that distributes the hydraulic fluid discharged from the hydraulic pump of the open circuit to the boom cylinder and the arm cylinder is provided to branch from a side closer to the hydraulic pump than the control valve in the open circuit.
Further, generally, a work machine such as a hydraulic excavator mounts a plurality of hydraulic pumps. Particularly, because in the case of a large-size hydraulic excavator, a working section itself to be driven becomes large in dimension, hydraulic fluid at a large flow rate becomes necessary for driving a hydraulic actuator that drives the working section. Then, Patent Literature 2 discloses prior art with a plurality of hydraulic pumps mounted thereon. In Patent Literature 2, the driving power of an engine is divided by a pump transmission for driving a plurality of hydraulic pumps.